The fiber-reinforced plastics industry, particularly the glass-reinforced polyester industry, is under pressure from environmental and safety regulators to reduce and eventually eliminate the use and production of volatile organic compounds. As a result, these industries are searching for methods that avoid the use of organic solvents, particularly in spraying applications. For example, the glass-reinforced polyester industry, which uses styrene in the manufacturing process, has begun to use styrene-suppressed resins. It is particularly desirable to avoid the use of styrene because it is highly tamable as well as a suspected carcinogen and environmental contaminant. These styrene-suppressed resins, however, include a significant amount of wax, which calls for special precautions, such as careful surface preparation to remove wax, to ensure good secondary bonds. Thus, alternative resin systems are needed for the preparation of fiber-reinforced plastic components.
Solvent-free and low-solvent two-component polyurethane systems are known and used in the production of coatings, resins, lacquers, pastes, and adhesives. For example, two-component lacquers based on 4,4'-diisocyanato diphenylmethane are used in quick-hardening coatings on concrete, steel, industrial floors, etc. Two-component polyurethane systems are also used in making molded parts. For example, in the reaction injection molding (RIM) process, a polyisocyanate and an isocyanate-reactive component, e.g., a mixture of a polyol and a chain extender or crosslinker having amine or hydroxyl groups, are mixed and injected, generally under high pressure, into a mold with subsequent rapid curing. These "two components," i.e., the isocyanate component and the isocyanate-reactive component, are highly reactive and therefore must be introduced into the mold very rapidly and perfectly mixed. Fiber-reinforced molded products can be made using the RIM process with a reinforcing mat placed in the mold. Generally inferior products result, however, because the RIM reactants react too quickly and become too viscous before they are able to completely impregnate the mat.
A more desirable process than reaction injection molding involves spraying. Few polyurethane compositions are available that can be sprayed, however, particularly to form fiber-reinforced molded products. This is because curing of polyurethane compositions are difficult to control such that fibrous reinforcement material, e.g., fiberglass, can be fully impregnated or wetted out.
Thus, a need exists for a method for the production of polyurethanes, particularly fiber-reinforced polyurethanes, that can be readily controlled, particularly with respect to the cure time and viscosity. Also, a need exists for a method for the production of polyurethanes that is substantially free of volatile organic compounds.